1. Field of the Invention
The present invention relates in general, to fiber optic sensors, and in particular, to fiber optic loop temperature sensor. The fiber optic loop temperature sensor finds particular utility in a temperature compensated self-referenced fiber optic microbend pressure transducer.
2. Description of the Related Art
Numerous applications exist for pressure measurement in high-temperature environments. These applications include pipe line pressure in petrochemical refineries, gas path pressure in aircraft engines and main steamline pressure in turbine generators. Traditional diaphragm type pressure transducers are not suited for these high-temperature applications for several reasons. First, the diaphragm material may creep and cause output offset error. Second, the strain or capacitance gages used to measure diaphragm deflection exhibit large non-repeatable and unpredictable changes in gauge outputs at temperatures greater than 300.degree. C. These changes are caused by such effects as alloy segregation, phase changes, selective oxidation, and diffusion. Ultimately, they lead to premature failure of the gage or lead-wire.
One attempt to eliminate the effects of diaphragm creep and hysteresis used dimensionally stable fused silica or other ceramic for the diaphragm material. Another attempt to enable measurement of diaphragm deflection at high temperatures employed fiber optic sensors with fused silica optical fiber.
While there are several fiber optic sensors suitable for sensing diaphragm deflection, most of these sensors have been configured for dynamic applications such as the detection of acoustic signals. Since pressure changes occur over long periods of time in most process applications, pressure transducers and manometers must be designed to measure static or dc pressure. Consequently, it is desirable that these devices have low drift and insensitivity to environmental changes other than pressure, for example, temperature.
In addition, there are practical problems associated with the design of fiber optic sensors to operate at elevated temperatures with adequate sensitivity within a transducer configuration exhibiting low drift and environmental insensitivity.
Microbend sensors exhibit zero offset as a function of temperature. The zero offset causes an apparent error in applications such as pressure transducers where the fiber optic microbend sensor is used to measure diaphragm deflection. It is known that microbend sensor offset is a linear, repeatable function of temperature. Thus, it is desirable that an independent, linear fiber optic measurement of temperature provide a signal which could be subtracted from the microbend sensor to provide a simple method for compensation of temperature offset.